Isolated resonator gyroscope with a drive and sense plate

ABSTRACT

The present invention discloses a resonator gyroscope comprising a vibrationally isolated resonator including a proof mass, a counterbalancing plate having an extensive planar region, and one or more flexures interconnecting the proof mass and counterbalancing plate. A baseplate is affixed to the resonator by the one or more flexures and sense and drive electrodes are affixed to the baseplate proximate to the extensive planar region of the counterbalancing plate for exciting the resonator and sensing movement of the gyroscope. The isolated resonator transfers substantially no net momentum to the baseplate when the resonator is excited.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation-in-part application claims the benefit under 35 U.S.C.§120 of the following co-pending and commonly-assigned U.S. utilitypatent application, which is incorporated by reference herein:

U.S. patent application Ser. No. 09/928,279, filed Aug. 10, 2001, andentitled “ISOLATED RESONATOR GYROSCOPE”, now U.S. Pat. No. 6,629,460,issued on Oct. 7, 2003.

STATEMENT OF GOVERNMENT RIGHTS

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 U.S.C. 202) in which the Contractor has elected to retain title.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to gyroscopes, and in particular toimproved resonator microgyroscopes and their manufacture. Moreparticularly, this invention relates to microgyroscopes operating withdrive and sense electrodes and a vibrationally isolated resonator.

2. Description of the Related Art

Gyroscopes are used to determine direction based upon the sensedinertial reaction of a moving mass. In various forms they are oftenemployed as a critical sensor for vehicles such as aircraft andspacecraft. They are generally useful for navigation or whenever it isnecessary to autonomously determine the orientation of a free object.

Older conventional gyroscopes were very heavy mechanisms, employingrelatively large spinning masses by current standards. A number ofrecent technologies have brought new forms of gyroscopes, includingoptical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes aswell as vibratory gyroscopes.

Spacecraft generally depend on inertial rate sensing equipment tosupplement attitude control. Currently this is often performed withexpensive conventional spinning mass gyros (e.g., a Kearfott inertialreference unit) or conventionally-machined hemispherical resonatorgyroscopes (e.g. a Litton hemispheric resonator gyroscope inertialreference unit). However, both of these are very expensive, large andheavy.

In addition, although some prior symmetric vibratory gyroscopes havebeen produced, their vibratory momentum is transferred directly to theirbaseplates or packages. This transfer or coupling admits externaldisturbances and energy loss indistinguishable from inertial rate inputand hence leads to sensing errors and drift. One example of such avibratory gyroscope may be found in U.S. Pat. No. 5,894,090 to Tang etal. which describes a symmetric cloverleaf vibratory gyroscope designand is hereby incorporated by reference herein. Other planar tuning forkgyroscopes may achieve a degree of isolation of the vibration from thebaseplate, however these gyroscopes lack the vibrational symmetrydesirable for tuned operation.

In addition, shell mode gyroscopes, such as the hemispherical resonatorgyroscope and the vibrating ring gyroscope, are known to have somedesirable isolation and vibrational symmetry attributes, however, thesedesigns are not suitable for or have significant limitations with thinplanar silicon microfabrication. The hemispherical resonator employs theextensive cylindrical sides of the hemisphere for sensitiveelectrostatic sensors and effective actuators, however its high aspectratio, 3D curved geometry is unsuitable for inexpensive thin planarsilicon microfabrication. The thin ring gyroscope while suitable forplanar silicon microfabrication lacks electrostatic sensor and actuatorsthat take advantage of the extensive planar area of the device.

Vibration isolation using a low-frequency seismic support is also known(e.g., U.S. Pat. No. 6,009,751, which is incorporated by referenceherein), however such increased isolation comes at the expense ofproportionately heavier seismic mass and/or lower support frequency.Both effects are undesirable for compact tactical inertial measurementunit (MU) applications.

Furthermore, the scale of previous silicon microgyroscopes (e.g., U.S.Pat. No. 5,894,090) has not been optimized for navigation gradeperformance resulting in higher noise and drift than desired. Thisproblem stems from a use of thin epitaxially grown silicon flexures todefine critical vibration frequencies that are limited to 0.1% thicknessaccuracy. Consequently device sizes are limited to a few millimeters.Such designs exhibit high drift due to vibrational asymmetry orunbalance and high rate noise due to lower mass which increases thermalmechanical noise and lower capacitance sensor area which increases rateerrors sensor electronics noise.

Scaling up of non-isolated silicon microgyros is also problematicbecause external energy losses will increase with no improvement inresonator Q and no reduction in case-sensitive drift. An isolatedcm-scale resonator with many orders of magnitude improvement in 3Dmanufacturing precision is required for navigation grade performance.

Conventionally machined navigation grade resonators such as inhemispherical or shell gyros have the optimum scale, e.g. 30 mm and 3Dmanufacturing precision and hence desirable drift and noise performance,however such gyros are expensive and slow to manufacture. Conventionallaser trimming of mechanical resonators can further improvemanufacturing precision to some degree, however this process is notsuitable for microgyros with narrow mechanical gaps and has limitedresolution, necessitating larger electrostatic bias adjustments in thefinal tuning process.

There is a need in the art for small gyroscopes with greatly improvedperformance for navigation and spacecraft payload pointing. There isalso a need for such gyros to be cheaper and more easily manufacturedwith greater 3D mechanical precision. There is still further a need forsuch gyros to have desirable isolation and vibrational symmetryattributes while being compatible with planar silicon manufacturing.Finally, there is a need for such gyros to have a compact, efficientdesign and optimized placement of drive and sense electrodes exploitingthe extensive planar area of the device. Finally, there is a need tomechanically trim the device to subatomic precision without producingdebris that may obstruct the capacitance gaps. The present inventionsatisfies all these needs.

SUMMARY OF THE INVENTION

Embodiments of the invention generally comprise an all-silicon, isolatedsymmetric vibratory gyroscope that is inexpensive to produce usingphotolithography and that can be scaled large enough (e.g.,approximately 20 mm resonator) to achieve the required performance.Combined with low-cost, closed-loop, analog control electronics, acomplete redundant inertial reference unit can be inexpensivelymanufactured, even when produced in small quantities. Further, whencombined with a low-power digital control electronicsapplication-specific integrated circuit (ASIC) for much largerquantities, a “golf ball” sized inertial navigation unit can beproduced. Such a compact, lightweight and inexpensive precision inertialreference unit can find a wide range of applications in military as wellas commercial products.

One embodiment of the invention provides an isolated symmetric planarresonator that can be fabricated with silicon photolithography. Thedesired isolation or reactionless vibration of the resonator is achievedby balancing the rocking momentum of the central inertia proof masselement against the rocking momentum of a counterbalancing plate ofcomparable rocking inertia. The proof mass and counterbalancing plateproduce substantially no net momentum transfer or reaction on a mountingbaseplate when a resonator differential rocking mode is excited. Anisolated resonator microgyroscope is thereby provided having no couplingof its sense or drive mode to baseplate or package motion except throughCoriolis accelerations when a differential rocking mode is internallydriven. In addition, the inertia distribution of the central proof masscan be designed to have more mass out of plane than the plate inembodiments of the present invention and hence higher angular gain(i.e., Coriolis reaction to inertial rate input). This can be arrangedusing a thicker plate or by bonding a central post. The resonator may beetched from a single thick silicon wafer including the proof mass orfrom a thin silicon wafer to which is bonded a post proof mass.

By comparison, previous vibratory gyroscopes with isolated resonatorshave relied on expensive conventional 3D machining and assembly toachieve isolation. For example, the 3D hemispherical resonator gyroscopemust be machined by conventional lathe or other time consumingsequential mass removal method. Simple micromachined gyroscopes withoutselfisolated design employ external low-frequency isolation mechanismsto gain a degree of isolation at the expense of increasing seismicsuspension mass and increased deflections due to gravity loads.Asymmetric tuning fork vibratory gyroscopes provide isolation about thedrive axis only and are subject to external disturbance about the outputsense axis. The original cloverleaf microgyroscope (e.g., U.S. Pat. No.5,894,090 to Tang et al.) is susceptible to external rockingdisturbances about its drive and output axes.

Significantly, embodiments of the present invention also use thecounterbalancing plate as the counter-electrode for the sense and driveelectrodes located on the baseplate below. Thus, the counterbalancingplate serves a dual role, forming an isolated resonator with the proofmass and providing a counter-electrode surface for the drive and senseelectrodes. To enhance the operation of the gyroscope, thecounterbalancing plate is structured with extensive planar surfaceregions allowing comparably extensive and effective electrodes forelectrostatic driving and sensing disposed on the baseplate below theresonator plate.

A typical embodiment of the invention comprises an isolated resonatorincluding a proof mass, a counterbalancing plate having an extensiveplanar region, and one or more flexures interconnecting the proof massand counterbalancing plate. A baseplate is affixed to the resonator bythe one or more flexures and drive and sense electrodes are affixed tothe baseplate proximate to the extensive planar region of thecounterbalancing plate for exciting the resonator and sensing movementof the gyroscope. The isolated resonator transfers substantially no netmomentum to the baseplate when the resonator is excited. In furtherembodiments isolation of the resonator is enhanced with additionalisolation flexures supporting the baseplate on a mounting frame.

A typical method of producing a resonator gyroscope of the inventioncomprises providing an isolated resonator including a proof mass, acounterbalancing plate having an extensive planar region, and one ormore flexures interconnecting the proof mass and counterbalancing plate.Next, sense and drive electrodes are affixed to the baseplate. Then, themethod further comprises affixing the resonator to a baseplate by theone or more flexures such that the drive and sense electrodes aredisposed proximate to the extensive planar region of thecounterbalancing plate and wherein the isolated resonator transferssubstantially no net momentum to the baseplate when the resonator isexcited.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 depicts a top view of an isolated resonator gyroscope of thepresent invention;

FIG. 2 depicts a side view of an isolated resonator gyroscope of thepresent invention in a displaced position;

FIG. 3 is a flowchart of a typical method of producing an isolatedresonator gyroscope of the invention;

FIG. 4 depicts a plan view of an exemplary reactionless planar resonatorgyroscope model; and

FIG. 5 illustrates a differential rocking mode about the X axis for anexemplary reactionless planar resonator gyroscope model; and

FIG. 6 illustrates a differential rocking mode about the Y axis for anexemplary reactionless planar resonator gyroscope model.

DETAILED DESCRIPTION INCLUDING PREFERRED EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings which form a part hereof, and which is shown, by way ofillustration, several embodiments of the present invention. It isunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

1.0 Overview

All vibratory gyroscopes employ a rotation sensing mechanical elementwhich is driven to oscillate in a first mode, i.e. the input or drivemode. A Coriolis acceleration of the element under rotation inducesenergy transfer from the input mode to a second mode, i.e. the output orsense mode. The second mode produces an excitation in the sense elementwhich is then detected. Optimum performance of a vibratory gyroscope isobtained when the drive and sense modes have the same resonant frequencyand a high Q factor. The response to the Coriolis acceleration is thenmechanically amplified by the Q factor of the resonance and providesimproved sensor performance. Closed loop control of the resonator sensemode (e.g. co-pending U.S. application Ser. No. 09/488,425, now U.S.Pat. No. 6,360,601, issued Mar. 28, 2003, which is hereby incorporatedby reference herein) provides Coriolis force-rebalance, sense modedamping and wide gyroscope bandwidth.

The present invention provides an affordable vibratory gyroscope withnavigation grade performance by means of a precision isolated symmetricplanar resonator of optimum scale that can be fabricated with siliconphotolithography from commercial double-side polished silicon waferswith low total thickness variation.

Embodiments of the present invention include a resonator comprising twobodies with transverse inertia symmetry about an axis aligned with aninput axis and elastically supported so that their axes of symmetry andcenters of mass coincide and together form two differential rockingmodes of vibration transverse to the axis of symmetry. The two bodiesare supported in a case having an inertial rate input axis and exhibitsubstantially equal frequencies distinct from other modes of vibration,mutually orthogonal and imparting substantially zero net momentum to thecase.

In the embodiments which follow, a first one of the bodies is a postproof mass, a second one of the bodies is a counterbalancing plate andthe case may include a baseplate. The counterbalancing plate includesplanar regions for reacting the drive and sense electrodes disposedunderneath the resonator on the baseplate. Other equivalent structuresand arrangements will be readily apparent to those skilled in the art.

A key principle of the present invention is that the drive and senseelectrodes are disposed to react with the counterbalancing plate. Toenhance the effect of this placement, the counterbalancing plate isformed with extensive planar regions to increase the surface areaavailable to react with the electrodes, improving their operation. Thedrive electrodes are aligned to drive a first one of the differentialrocking modes to vibrate. The sense electrodes are aligned to sense themotion of the second differential rocking mode induced by Coriolisaccelerations resulting from the inertial rate input and internallydriven differential rocking motion about the first mode axis.

2.0 Exemplary Resonator Gyroscope Embodiment

FIG. 1 depicts a top view of a reactionless resonator gyroscope 100embodiment of the present invention. The gyroscope 100 comprises abaseplate 106 and a unique resonator 124 which includes a post inertialproof mass 102 and a counterbalancing plate 104. The counterbalancingplate 104 has a rocking inertia substantially comparable to that of theinertial proof mass 102 and these two bodies are interconnected andinteract as described above. The counterbalancing plate 104 and centralproof mass 102 are coupled to the baseplate 106 at four mounting points126 and interconnected to each other by way of flexures 108. Theprincipal axes of concern which will be referenced throughout thespecification are the X axis 110, the Y axis 112 and the Z axis 114(which is directed out of the page of FIG. 1). Alternately, thecounterbalancing plate 104 can also be designed in any other shape, suchas a circular or other arbitrary shape so long as the two bodies(inertial proof mass 102 and counterbalancing plate 104) interact aspreviously described.

FIG. 2 depicts a side view of a reactionless resonator gyroscope 100 ofthe present invention in a displaced position. The gyroscope is showndisplaced about the X axis 110. The mechanical assembly comprises acentral inertial proof mass 102 element interconnected to an outercounterbalancing plate 104 and affixed to a baseplate 106 via elasticbeam flexures 108 attached at the four mounting points 126. In oneembodiment, the counterbalancing plate 104, flexures 108 and supportplate 118 for the central inertial proof mass 102 can bephotolithographically etched-through from the same double-side polishedcrystal silicon wafer to produce a precision planar resonator gyroscope.

The axisymmetric resonator is coupled to a baseplate 106 such that theaxisymmetric counterbalancing plate 104 can freely vibrate against theaxisymmetric central proof mass 102 with counterbalanced oscillatoryrocking motion and results in a distinct differential rocking mode withsubstantially no momentum transfer to or net reaction on the baseplate106.

The baseplate 106 may be a relatively thick silicon plate of rigidmaterial. Such a thick rigid baseplate 106 can be directly bonded to theremainder of the gyroscope in a vacuum package. Alternatively, a moreflexible thin baseplate 106 may be used to reduce cost and ease assemblywith standard wafer processing equipment. Common elasticity in theresonator flexures 108 such as in the attachment to the baseplate 106and finite inertia of the baseplate provides inherent separation of thedifferential rocking mode frequency from the common rocking mode ofvibration. The singular attribute of any of these arrangements is thatany external motion of the gyroscope package cannot excite differentialrocking motion of the resonator, unless such motion is first internallydriven and only then by virtue of Coriolis accelerations due to rotationof the gyroscope about the input axis or axis of inertial symmetry.

The proof mass 102 can be constructed in various forms, however theinertial distribution of the central proof mass is designed to havesignificantly more mass out of plane than in plane and hence highangular gain, or Coriolis reaction to inertial rate input with drivenrocking motion of the resonator 124. To achieve this, the proof mass 102comprises a vertical post portion 116 (elongated along the Z axis 114).The post portion 116 is disposed on a small central support plateportion 118 (in the X-Y plane). The post portion 116 and support plateportion 118 can be formed together from a thick silicon wafer formanufacturing ease as previously mentioned. Alternately, the proof mass102 can be bonded as a separable central post portion 116 to the supportplate portion 118 formed from a thin silicon wafer.

The common flexures 108 that connect the central proof mass 102 to boththe moving counterbalancing plate 104 and to the fixed baseplate 106 canbe photolithographically machined from a single silicon wafer of precisethickness along with the planar outer counterbalancing plate 104 and acentral plate portion 118 for mounting a post as the vertical portion116 as the proof mass 102. The precision planar construction, largeplanar areas for electrostatic sense and control and favorable massdistribution provide symmetric rocking motion for sensitive tunedvibratory gyroscope operation with ideal mechanical isolation fromexternal disturbances.

2.1 Sense and Drive Counterbalancing Plate

As previously described, embodiments of the present invention utilizethe counterbalancing plate to hold the sense and drive electrodes,instead of the “cloverleaf” of the related art (e.g., U.S. Pat. No.5,894,090).

Electrostatic driving and sensing can be implemented with the drive andsense electrodes 120A, 120B (collectively referred to as electrodes 120)affixed to the baseplate 106 beneath the large planar surfaces of thecounterbalancing plate 104. See FIG. 1. The large surface area of thecounterbalancing plate 104 is used to react with the driving and sensingelectrodes 120. In general, the extensive planar electrode 120 regionsare formed on the baseplate 106 beneath the counterbalancing plate 104.The counterbalancing plate 104 structure extends toward the centralproof mass 102 beyond the attachment points 122 of flexures 108 to thecounterbalancing plate 104 as shown in FIG. 1 to maximize the usefularea. Thus, the gap between the proof mass 102 and the counterbalancingplate 104 is reduced and the counterbalancing plate 104 obtains a moreplate-like configuration.

Also as shown in FIG. 1, a typical arrangement of the drive and senseelectrodes 120A, 120B is for the drive electrodes 120A to be disposednearer to the proof mass 102 than the sense electrodes 120B; one driveelectrode 120A and one sense electrode 120B each are disposed on thebaseplate 106 under each quarter segment of the counterbalancing plate104. This improves the overall sensitivity of the microgyro 100 as thesense electrodes 120B obtain a larger surface area and the gaps aroundthe periphery of the counterbalancing plate 104 undergo largerdisplacements relative to the baseplate 106. Other electrode 120patterns can also be used as well, however. For example, the electrodes120 can be interwoven.

In addition, to allow greater planar area of the counterbalancing platefor the drive and sense electrodes, the flexures 108 can be supported bythe baseplate 106 off-center. As shown in FIG. 1, the baseplate 106supports the flexures 108 at points nearer to the proof mass 102 thanthe attachment points 122 to the counterbalancing plate 104.

2.2 Resonator Flexures and Integral Baseplate Isolation

Many configurations of the isolated resonator are possible by varyingthe resonator flexure 108 layout. Different embodiments of the presentinvention can also employ the resonator flexures 108 arranged indifferent patterns. For example, as shown in FIG. 1, the resonatorflexures 108 are arranged to extend in a radial direction from the proofmass 102 to the counterbalancing plate 104. In other embodiments, theflexures 108 can also be arranged to extend around a perimeter of theproof mass 102 (still interconnecting the proof mass 102 and thecounterbalancing plate 104). In addition, hybrids are also possible withsome flexures 108 extending in a radial direction and others extendingaround a perimeter.

Furthermore, embodiments of the present invention can also incorporateintegral vibration isolation to the baseplate 106 to further improvedifferential and common rocking mode frequency separation and vibrationisolation of the resonator 124. In these embodiments, a mounting frame128 is attached to the baseplate 106 through one or more isolationflexures 130. In this case, the baseplate 106 isolation of the resonator124 is primarily considered with respect to the mounting frame 128 withthe baseplate 106 serving as an additional isolating element. Forexample, the baseplate 106 isolation flexure 130 width and/or length canbe set to attenuate axial or rocking vibrations above 500 Hz from thecase. In the embodiment shown in FIGS. 1 and 2 the isolating flexures130 extend around a perimeter of the baseplate 106, attached to thebaseplate 106 at a first end and the mounting frame 128 at a second end(illustrating flexures extending around a perimeter as described above).As previously discussed with respect to the resonator flexures 108,radial flexure layouts and hybrids can similarly be used for theisolation flexures 130.

In addition, other implementations of the invention are also possible inwhich the flexures 108, 130 are not necessarily discrete elements butrather built into the counterbalancing plate 104, proof mass 102,baseplate 106 and/or mounting frame 128. In any case, the essentialrequirement is that there be substantially no net reaction or momentumtransfer to the baseplate.

3.0 Producing a Practical Isolated Resonator Gyroscope

FIG. 3 is a flowchart of a typical method 300 producing an isolatedresonator gyroscope of the invention. The method comprises providing anisolated resonator 124 including a proof mass 102, a counterbalancingplate 104 having an extensive planar region, and one or more flexures108 interconnecting the proof mass 102 and counterbalancing plate 104 atblock 302. Next at block 304, sense and drive electrodes 120 are affixedto the baseplate 106. At block 306, the method 300 further comprisesaffixing the resonator 124 to a baseplate 106 by the one or moreflexures 108 such that the drive and sense electrodes are disposedproximate to the extensive planar region of the counterbalancing plateand wherein the isolated resonator 124 transfers substantially no netmomentum to the baseplate 106 when the resonator 124 is excited. Forexample, the resonator 124 can be etched from conductive doped siliconand rigidly and conductively bonded to the baseplate using gold—goldthermo-compression bonding or gold-silicon eutectic bonding.

Providing the resonator 124 may also comprise etching the entire proofmass 102 and counterbalancing plate 104 from a single silicon wafer oretching only a plate portion 118 and the counterbalancing plate 104 fromthe silicon and bonding on a separate vertical post portion 116 as theproof mass 102. A gold—gold thermo-compression bond for a silicon postor anodic bond for a pyrex post can be used with critical precisionbonding surfaces and dimensions defined by planar polishing.

Conventional silicon processing can be employed for both resonator andbaseplate wafers. The resonator can be manufactured from a doped,double-side polished, plain 500-micron silicon wafer with preferablyless than 1 micron total thickness variation (TTV) specification. Thebaseplate can be a commercial 500-micron silicon wafer with a 26 to 50micron epitaxial silicon layer or silicon-on-insulator (SOI) layer forthe gap spacer. Alternatively, a plain wafer with an etched electrodewell can be used. Definition of the resonator flexures 108 and baseplate106 flexures can be performed with commercial deep reactive ion etchingequipment. Alignment and attachment of the inertial proof mass 102 withvisual optical precision is sufficient, based on FEM analysis andexperience with related art MEMS cloverleaf gyros.

The principle of operation of the resonator gyroscope is that the proofmass rocks against the counterbalancing plate with equal an oppositemomentum. Thus, an exemplary gyroscope 100 can be made from a planar 550micron thick silicon wafer, the resonator 124 comprises a 7.5 mm×7.5 mmplate (central plate portion 118) with 10.5 mm×3 mm×3 mm silicon posts(proof mass 102) bonded to both sides. The rocking inertia of the proofmass 102 can be approximately I_(t)=0.014 kg-mm² and the axial inertiaapproximately I_(Z=)0.0060 kg-mm². The angular gain for Coriolis sensingis approximately 1-I_(Z)/I_(t)/2˜1 (comparable to an ideal Foucaultpendulum). The angular gain for the flat plate is approximately 0 so theeffective angular gain for the gyroscope is approximately 0.5,comparable to the typical value of 0.3 for a hemispherical or shellresonator. For reactionless operation, the rocking inertia of thecounterbalancing plate 104 substantially matches that of the proof mass102, I_(t). The center of mass of the proof mass 102 andcounterbalancing plate 104 are thus coincident at the center of theresonator 124. Acceleration of the gyroscope 100 thus producessubstantially no excitation of the differential rocking modes.

The elastic flexures 108, used to attach the central plate portion 118and counterbalancing plate 104 to the rigid baseplate 106, can haveapproximately a 0.55 mm square cross section, fabricated byphotolithography. The attachment and lengths of the elastic elements 108to the baseplate 124 can be determined through a finite elementanalysis, such as described hereafter.

An analysis of the free modes of vibration of the resonator 124 attachedto a 3 mm thick 3 inch diameter silicon wafer identified two, degeneratecounter-rocking modes of the counterbalancing plate 104 against theproof mass 102 at approximately 4,295.77 Hz. The momentum transmitted tothe baseplate 106 is found to be approximately 1/150 that of a rockingmomentum in the proof mass 102. By comparison, with a locked plate orframe as in designs such as that of U.S. Pat. No. 5,894,090, all of therocking momentum would be transmitted to the baseplate wafer. Furtherexamination of the finite element model results reveals how the balanceis achieved about the X axis of a baseplate 106 in this particular case:first, there is substantially no net in-plane torsion or normal force ateach of the baseplate 106 support points along the X axis; second, theresidual in-plane torsion reactions at each of the flexure 108attachments along the Y axis are counterbalanced at any point in thebaseplate 106 by the net moment due to the normal force also acting atthese attachments. This demonstrates an approximately 150 foldimprovement in mechanical isolation without resort to low frequencyseismic isolation methods. The perfection of the balance and isolationis limited only by planar geometric design and fabrication precision andnot by the size of the baseplate 106 mass and its suspension frequencyfrom the gyro case.

The present invention is appropriate for navigation grade vibratorygyroscopes in rugged environments. The isolation of the two rockingmodes from rigid baseplate motion ensures that modal damping andassociated gyroscope rate drift will be determined primarily by thelosses within the precision machined silicon resonator and not by themuch less precise external packaging losses. The inherent high Q of bulkcrystal silicon and the excellent symmetry which has been demonstratedthrough photolithography of precision double-side polished siliconwafers at mesoscale can be exploited with the present invention toachieve excellent navigation grade vibratory gyroscope performance atlow cost.

It should also be understood that to achieve the full potential of lowdrift and noise performance using this isolated resonator principle willrequire even high final 3D mechanical precision than afforded by theprecise wafer polishing, through-etched silicon micromachining and highquality silicon bonding outlined above. This can be preferablyaccomplished with this design by focused ion beam trimming, after theassembly of the resonator onto its baseplate, of the dimensions of theelastic beam elements or the mass elements of the resonator, e.g. plateor post. This post assembly trimming can take advantage of the highlysensitive built in capacitive sensors to increase the degree of initialisolation and tuning to subatomic precision. Electrostatic bias trimmingto modify the overall modal stiffness with the built in capacitanceelectrodes or special purpose electrodes can be used to maintain thisisolation and tuning after vacuum packaging and at varying temperaturesthroughout the life of the gyroscope.

4.0 Isolated Resonator Gyroscope Model

FIG. 4 depicts a view of an isolated planar resonator gyroscope model.As previously detailed, the mechanical assembly comprises a resonator124 and baseplate 106. The resonator 124 comprises a central inertiaproof mass 102 element attached to a planar counterbalancing plate 104and to a baseplate 106 via elastic beam flexure elements 108. The planarcounterbalancing plate 104 inertia matches the proof mass 102 rockinginertia. Both can be fabricated from polished crystal silicon so thattheir inertias can be set approximately equal by geometric design.

Differential rocking of a post proof mass 102 against the planarcounterbalancing plate 104 provides the two desirable degenerate,reactionless modes of vibration for sensing inertial rate with highmechanical gain. The high angular gain of the elongated post proof mass102 versus the counterbalancing plate 104 provides high sensitivity toCoriolis force and hence an inertial rate normal to plane. The baseplatecarries the short (approximately 15-micron) pillars for attachment ofthe resonator flexures 108 and provision of the capacitance gap forbaseplate electrodes to drive and sense the vibration of the planarcounterbalancing plate 104 of the resonator 124.

The baseplate 106 also provides the integral vibration isolation andouter mounting frame for packaging. Preferably, the resonator is etchedfrom the same double-side polished crystal silicon wafer without thevertical portion 116 of the inertia proof mass 102. In addition, thebaseplate 106 is also preferably etched from a silicon wafer withstandard semiconductor processing. Since the baseplate 106 does not moveby virtue of the disclosed resonator isolation method its mechanicalprecision is less critical.

FIGS. 5 and 6 illustrate the key differential rocking modes about the Xand Y axes, respectively. In a practical simulation, the model obtainsmomentum isolation of approximately 10⁻⁴ with approximately 1 micronerror in beam length. The resulting frequency split was approximately0.2 Hz. Current micromachining error is estimated to be less than 100 nmover 2 cm in the lateral X and Y directions. Error of less than 10 nm on500 micron thickness is estimated in the Z direction as confirmed byoptical metrology. Resonators have been machined with measured rockingmode frequency splits less than 0.1 Hz, without posts.

The FEM analysis used to generate FIGS. 4-6 comprised simple beamelements for the key resonator flexures 108 and baseplate isolationflexures 130, hexahedral solid elements for the inertia proof mass 102and rigid mass elements for the planar counterbalancing plate 104,central support plate portion 118 and baseplate 106. Construction linesfor plot only elements defining the counterbalancing plate 104 of theresonator 124 are visible in the FIGS. 4-6 but have no structuralsignificance. A refined model using plate elements for thecounterbalancing plate 104 and central baseplate 106, and solidhexahedral elements for the inertia proof mass 102, can be used for amore precise geometry definition prior to final photo-mask generation.

CONCLUSION

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto. The above specification, examples and dataprovide a complete description of the manufacture and use of theinvention. Since many embodiments of the invention can be made withoutdeparting from the scope of the invention, the invention resides in theclaims hereinafter appended.

1. A resonator gyroscope, comprising: an isolated resonator including: aproof mass; a counterbalancing plate having an extensive planar region;and one or more flexures interconnecting the proof mass andcounterbalancing plate; a baseplate affixed to the resonator by the oneor more flexures; and drive and sense electrodes affixed to thebaseplate proximate to the extensive planar region of thecounterbalancing plate for exciting the resonator and sensing movementof the gyroscope; wherein the isolated resonator transfers substantiallyno net momentum to the baseplate when the resonator is excited.
 2. Theresonator gyroscope comprising: an isolated resonator including: a proofmass; a counterbalancing plate having an extensive planar region; andone or more flexures interconnecting the proof mass and counterbalancingplate; a baseplate affixed to the resonator by the one or more flexures;a mounting frame attached to the baseplate through at least oneisolation flexure: and drive and sense electrodes affixed to thebaseplate proximate to the extenstive planar region of thecounterbalancing plate for exciting the resonator and sensing movementof the pyroscope: wherein the isolated resonator transfers substantiallyno net momentum to the baseplate when the resonator is excited.
 3. Aresonator gyroscope comprising: an isolated resonator including: a proofmass; a counterbalancing plate having an extensive planar region; andone or more flexures interconnecting the proof mass and counterbalancingplate, wherein each of the one or more flexures in a beam flexureattached to the proof mass at a first end and the counterbalancing plateat a second end; a baseplate affixed to the resonator by the one or moreflexures; and drive and sense electrodes affixed to the baseplateproximate to the extensive planar region of the counterbalancing forexciting the resonator and sensing movement of the gyroscope; whereinthe isolated resonator transfers substantially no net momentum to thebaseplate when the resonator is excited.
 4. The resonator gyroscopecomprising: an isolated resonator including: a proof mass, wherein theproof mass comprises a vertical protion, the vertical portion is aseparate element bonded to a central plate portion, and the separateelement comprises sapphire; a counterbalancing plane having an extensiveplanar region: and one or more flexures interconnecting the proof massand counterbalancing plate; a baseplate affixed to the resonantor by theone or more flexures; and drive and sense electrodes affixed to thebaseplate proximate to the extensive planar region of thecounterbalancing plate for exciting the resonator and sensing movementof the gyroscope; wherein the isolated resonator transfers substantiallyno net momentum to the baseplate when the resonator is excited.
 5. Theresonator gyroscope of claims 1, 2, 3 or 4 wherein the proof mass andcounterbalancing plate each have a center mass and transverse inertiasymmetry about an axis are substantially coincident and the proof massand counterbalancing plate together form two differential rocking modesof vibrations transverse to the axis with substantially equalfrequencies.
 6. The resonator gyroscope of claims 1, 2, 3 or 4, whereinthe baseplate is rigid.
 7. The resonator gyroscope of claims 1, 2, 3 or4, wherein the baseplate is flexible.
 8. The resonator gyroscope ofclaims 1, 2, 3 or 4, wherein the counterbalancing plate has a rockinginertia substantially compatable to that of the proof mass.
 9. Theresonator gyroscope of claims 1, 2, 3 or 4, wherein the one or moreflexures are integral to the counterbalancing plate.
 10. The resonatorgyroscope of claims 1, 2, 3 or 4, wherein the proof mass,counterbalancing plate and baseplate are machined from silicon.
 11. Theresonator gyroscope of claim 3, wherein each beam flexure is attached tothe baseplate off center and nearer to the proof mass than an attachmentpoint to the counterbalancing plate.
 12. The resonator gyroscope ofclaim 4, wherein the one or more flexure are integral to the centralplate portion.
 13. The resonator gyroscope of claim 4, wherein thecentral plate portion, one or more flexures and the counterbalancingplate are produced by through- etching a precision-polished siliconwafer.
 14. A method of producing a resonator gyroscope, comprising thesteps of: providing an isolated resonatory including: a proof mass; acounterbalancing plate having an extensive planar region: and one ormore flexures interconnecting the proof mass and counterbalancing plate;affixing drive and sense electrodes to the baseplate: and affixing theresonator to a baseplate by the one or more flexures such that the driveand sense electrodes are disposed proximate to the extensive planarregion of the counterbalancing plate, wherein the sense electrodes aredisposed around the periphery of the counterbalancing plate: wherein theisolated resonatory transfers substantially no net momentum to thebaseplate when the resonatory is excited.
 15. A method of producing aresonator gyroscope, comprising the steps of: providing an isolatedresonatory including: a proof mass; a counterbalancing plate having anextensive planar region; and one or more flexures interconnecting theproof mass and counterbalancing plate; affixing drive and senseelectrodes to the baseplate; affixing the resonator to a baseplate bythe oe oe more flexures such that the drive and sense electrodes aredisposed proximate to the extensive planar region of thecounterbalancing plate; and providing a mounting frame attached to thebaseplate through at least one isolation flexure; wherein the isolatedresonator transfers substantially no net momentum to the baseplate whenthe resonator is excited. separate element bonded to the central plateportion.
 16. A method of producing a resonator gyroscope, comprising thesteps of: providing an isolated resonator including: a proof mass; acounterbalancing plate having an extensive planar region: and one ormore flexures interconnecting the proof mass and counterbalancing plate,wherein each of the one or more flexures is a beam flexure attached tothe proof mass at a first end and the counterbalancing at a second end;affixing drive and sense electrodes to the baseplate: and affixing theresonator to a baseplate by the one or more flexures such that the driveand sense electrodes are disposed proximate to the extensive planarregion of the counterbalancing plate; wherein the isolated resonatortransfers substantially no net momentum to the baseplate when theresonator is excited.
 17. A method of producing a resonator gyroscope,comprising the steps of: providing an isolated resonator including: aproof mass, wherein the proof mass comprises a vertical portion, thevertical portion is a separate element bonded to a central plateportion, and the separate element comprises sapphire; a counterbalancingplate having an extensive planar region; and one or more flexuresinterconnecting the proof mass and counterbalancing plate; affixingdrive and sense electrodes to the baseplate; and affixing the resonatorto a baseplate by the o ne or more flexures such that the drive andsense electrodes are disposed proximate to the extensive planar regionof the counterbalancing plate; wherein the isolated resonator transferssubstantially no net momentum to the baseplate when the resonator isexcited.
 18. The method of claims 14, 15, 16 and 17 wherein the proofmass and counterbalancing plate have a center of mass and transverseinertia symmetry about an axis that are substantially coincident and theproof and the counterbalancing plate together from two differentialrocking modes of vibration transverse to the axis with substantiallyequal frequencies.
 19. The method of claim 14, 15, 16 and 17, whereinthe baseplate is rigid.
 20. The method of claim 14, 15, 16 and 17,wherein the baseplate is flexible.
 21. The method of claim 14, 15, 16and 17, wherein the counterbalancing plate has a rocking inertiasubstantially compatible to that of the proof mass.
 22. The method ofclaim 14, 15, 16 and 17, wherein the one or more flexures are producedintegral to the counterbalancing plate.
 23. The method of claim 14, 15,16 and 17, wherein the steps of producing and affixing comprisemachining the proof mass, counterbalancing plate and baseplate fromsilicon.
 24. The method of claim 16, wherein each beam flexure isattached to the baseplate off center and nearer to the proof than anattachment point to the counterbalancing plate.
 25. The method of claim17, wherein the one or more flexure are integral to the central plateportion.
 26. The method of claim 17, wherein the central plate portionone or more flexure and the counterbalancing plate are produced bythrough etching a precision-polished silicon wafer.